The general concept of utilizing electrically writable and erasable phase change materials (i.e., materials which can be electrically switched between generally amorphous and generally crystalline states) for electronic memory applications is well known in the art and as is disclosed, for example, in U.S. Pat. No. 3,271,591 to the original inventor Ovshinsky, et al issued Sept. 6, 1966 and in U.S. Pat. No. 3,530,441 of Ovshinsky, et al issued Sept. 22, 1970, both assigned to the same assignee as the present invention, both disclosures of which are incorporated herein by reference.
As disclosed in the aforementioned Ovshinsky patents, such phase change materials can be electrically switched between two different structural states of generally amorphous and generally crystalline local order or between different detectable states of local order across the entire spectrum between the completely amorphous and the completely crystalline states. That is, the switching of such materials is not required to take place between completely amorphous and completely crystalline states but rather can be in incremental steps reflecting changes of local order to provide a "gray scale" represented by a multiplicity of conditions of local order spanning the spectrum between the completely amorphous and the completely crystalline states. The materials described can also be switched between only two structural states of generally amorphous and generally crystalline local order to accommodate the storage and retrieval of binary encoded information.
The aforementioned pioneering Ovshinsky electrically erasable phase change memories were useful for many applications at the time of the original introduction thereof and were utilized in a number of commercially significant applications. However, because further development of that early technology was not possible due to the lack of the necessary financial resources to carry the same forward, subsequent developments in other fields of solid state, electronic memories and in other types of memories in general, such as those utilizing magnetic and optical media, displaced that early electrically erasable phase change technology in the marketplace.
As a result of the aforementioned lack of ongoing financial support, there were, prior to the instant invention, several limitations in the application of the electrically erasable memory of the early Ovshinsky phase change materials which have prevented their widespread use as solid state electrical replacements for archival, mass storage and cache memory elements currently dominating the computer field.
For example, in a typical personal computer there are four tiers of memory. Archival information is stored in inexpensive, slow, high storage capacity, non-volatile devices such as magnetic tape and floppy disks. This information is transferred as it is needed to faster and more expensive, but still non-volatile, hard disk memories. Information from the hard disks is transferred, in turn, to the still more expensive, faster, volatile system memory which uses semiconductor dynamic RAM (DRAM) devices. Very fast computers even transfer forth and back small portions of the information stored in DRAM to even faster and even more expensive volatile static RAM (SRAM) devices so that the microprocessor will not be slowed down in the computations by the time required to fetch data from the DRAM. Transfer of information among the tiers of the memory hierarchy occupies some of the computer's power and this need for "overhead" reduces performance, and, of course, it results in additional complexity in the computer's architecture. The current use of the hierarchal structure, however, is dictated by the price and performance of available memory devices and the need to optimize computer performance while minimizing cost.
The aforementioned limitations which have heretofore prevented the widespread use of the prior art Ovshinsky-type electrical memory materials as a replacement for present computer memory applications, such as specifically and without limitation, tape, optical, hard disk drive, solid state disk flash, DRAM, SRAM, and socket flash memory media, have included: (1) the relatively slow (by present standards) electrical switching speed which characterized such prior art phase change materials, particularly when switched in the direction of greater local order, i.e., in the direction of increasing crystallization; (2) the relatively high energy which had to be applied to memory elements fabricated therefrom in order to initiate the detectable change in local order, and (3) high cost per megabyte of stored information, particularly in comparison to hard disk storage. More specifically, it was necessary to apply a relatively high amount of energy in order to obtain detectable changes in the chemical and/or electronic bonding configurations of the chalcogenide material from which the element was fabricated as said element is set between one state and the other.
For example, the switching times of such prior art phase change materials were typically in the range of a few milliseconds for the set time, the time in which the material was switched from the amorphous state to the crystalline state; and approximately a microsecond for the reset time, the time in which the material was switched from the crystalline state back to the amorphous state. The electrical energy required to switch such prior art materials was relatively high, typically measured to be in the range of about a microjoule. It should be noted that this amount of energy must be deliverable to each of the memory elements in the solid state matrix of rows and columns of memory cells. Such high energy levels translate into high current carrying requirements which each of the isolation/address devices associated with each of the discrete memory elements must be capable of rectifying and carrying. If one takes into consideration the aforementioned energy requirements necessary for the successful operation of such a prior art matrix array, the choices of rectifying elements to which an artisan skilled in the art would be limited to very large single crystal isolation devices, thereby making the use of micron scale lithography and hence a high packing density of memory elements impossible. The low bit densities of the prior art matrix arrays would therefore have a high cost per megabyte of stored information.
By effectively narrowing the distinction in price and performance between archival, non-volatile mass memory and fast, volatile system memory, the memory elements of the instant invention provide for the creation of novel, non-hierarchal "universal memory system". Essentially all of the memory in the system can be low cost, archival and fast. As compared to original Ovshinsky-type phase change electrical memories, the memory materials described herein provide over 6 orders of magnitude faster programming time (less than 10 nanoseconds) and uses extraordinarily low programming energy (less than 50 picojoules) with demonstrated long term stability and cyclability (in excess of 20 million cycles). Also, experimental results indicate that additional reductions in element size can increase switching speeds and cycle life.
The concept of utilizing the aforementioned Ovshinsky-type phase change materials in non-erasable or non-reversible, write-once electrically programmable memories is also disclosed by the prior art. This type of electrically programmable phase change memory is described, for example, in U.S. Pat. No. 4,499,557 of Holmberg, et al. issued Feb. 12, 1985 and U.S. Pat. No. 4,599,705 of Holmberg, et al. issued Jul. 8, 1986, both of which are assigned to the same assignee as the present invention and the disclosure of which are incorporated herein by reference. These Holmberg, et al. patents include tetrahedrally chemically bonded materials such as carbon, silicon and germanium and alloys thereof as phase change materials which are utilized in a non-reversible or non-resettable mode. Such materials are disclosed as having, for example, characteristics which require threshold setting voltages of up to 10 volts, currents up to 25 milliamps and setting times of up to 100 microseconds. Thus, the set power required is up to 250 milliwatts with corresponding set times of up to 100 microseconds.
Due to the lack of ongoing financial support for further development, the general class of chalcogenide memory materials were not optimized and accordingly failed to find widespread use in reversible or electrically erasable memory applications. In marked distinction thereto, other types of solid state electrical memories have been developed to provide substantially faster switching times and substantially lower set and reset energies. Because of the improvements in time and energy, said other types of solid state, electronic memories now enjoy some limited acceptance in the marketplace. These other forms of memories typically employ several solid state microelectronic circuit elements for each memory bit, as many as three or four transistors per bit, for example, in some memory applications. The primary memory elements in such solid state memories, such as DRAM, are typically floating gate field effect transistor devices which hold a charge on the gate of a field effect transistor to store each memory bit. Since this charge can leak off with the passage of time, the storage of information is not truly non-volatile as it is in the phase change media of the prior art where information is stored through changes in the actual atomic configuration or electronic structure of the chalcogenide material from which the elements are fabricated.
In contrast to DRAM and SRAM volatile memory devices and unlike other non-volatile EEPROM devices, such as floating gate structures, no field effect transistor devices are required in the electrical memory devices of the instant invention. In fact the electrically erasable, directly overwritable memory elements of the instant invention represent the simplest possible electrical memory device to fabricate, comprising only two electrical contacts to a monolithic body of thin film chalcogenide material and a semiconductor diode for isolation. As a result, very little chip "real estate" is required to store a bit of information, thereby providing for a configuration of inherently high density memory chips. Further, and as will be detailed hereinafter, additional increases in information density can be accomplished in the memory elements of the instant invention through the use of multibit storage in each discrete memory cell.
Solid state, electronic memories presently in use are relatively expensive to manufacture, the cost being typically about twice the cost per bit of storage capacity in relation to magnetic disk storage. On the other hand, those solid state, electronic memories provide certain advantages over magnetic disk memories in that solid state memories have no moving parts, require much less electrical energy to operate, are easy to transport and store and are more versatile in the adaptability thereof for use with portable computers and other portable electronic devices. As a matter of fact, hard drive manufacturers are forecasting rapid growth in the use of ever smaller hard drives and eventually solid state memory storage in the portable computer field. In addition, such solid state memories are usually true random access systems as opposed to disk types which require physical movement of the disk head to the proper data track for accessing the desired memory location.
However, in spite of such advantages of solid state electrically erasable memories, the substantially higher costs thereof have prevented them from enjoying a substantial share of the market now dominated by magnetic disk type memory systems. Although solid state memories based on chalcogenide memory materials have shown potential for being manufactured at reduced costs, the overall performance parameters available from such transistor-based ferroelectric systems as previously known in the prior art have been inadequate to gain widespread use as replacements for magnetic disk systems or other solid state memory systems of the type described above.
Simply stated, no solid state memory system developed prior to the system disclosed by the instant invention, regardless of the materials from which it was fabricated, has provided all the advantages of being low cost, easily manufacturable, electrically writable and directly erasable (overwritable) low input energies, gray scale (allowing multibit in a single cell), non-volatile, and very high packing density. Accordingly, it is clear that the memory system described in the paragraphs which follow hereinafter will find immediate widespread use as a universal replacement for virtually all types of computer memory currently in the marketplace. Further, due to the ability of the novel memories of the instant invention to be fabricated in all thin-film format, three-dimensional arrays are now possible for high speed, high density neural network and artifical intelligence applications. The applicability of this technology to neural networks and artificial intelligence demands a multi-layer, three-dimensional approach.
As is readily apparent from the discussion above, the quantitative changes in switching speed and energy requirements of the memories of the instant invention in comparison to the phase change memories of the prior art is demonstrative evidence that the memory materials of the instant invention define an entirely new class of memory materials. Moreover, the wide dynamic range coupled with the multibit storage capabilities find no analog in the field of optical memory materials.